Did you replace your windows and then find steamed-up glass and mould in the corners the following winter? It is not rare, and it rarely means the windows are bad. It means the house lost the gaps through which damp air used to escape unnoticed. This guide explains why, what Portuguese law says about air inlets when windows are replaced, how to measure the problem and what to ask the installer.
Updated 4 November 2026. General information, not a substitute for assessment by a technician.
The essentials
- New windows close better. The UK Department of Health and Social Care (DHSC) warns that improving airtightness may require better ventilation to avoid poor air quality and condensation.
- Portaria n.º 138-I/2021 (a Portuguese government ordinance) asks that when the glazed envelope of main rooms (bedrooms and living room) is replaced in a renovation that is not a major one, air inlets are provided in the façade: 60 cm² of free area or self-regulating vents.
- The Swiss Federal Office of Public Health (FOPH) gives 30% to 50% relative humidity in the heating season and says to air the home at least three times a day.
- Mould only on the window sealant joints is small. Mould in several rooms or on walls needs an investigated cause.
- Before you pay, ask in writing whether the window includes an air inlet and of what kind.
Why mould appears after you change the windows
In a house with old windows, air comes in and goes out through the gaps. That is free ventilation, but it is cold. Showering, cooking and breathing all put water vapour into a home. The FOPH explains that if the vapour cannot leave, mould patches eventually appear in slightly colder places such as windows, frames and outside walls.
A new window seals those gaps. The vapour stays inside and goes to the coldest spots. The FOPH adds that the cause is not always too little ventilation: dirty extractors, very small windows or tilt-and-turn windows can also ventilate badly.
This links to other guides: a cold corner is a thermal bridge problem, and a bedroom closed at night is the classic case of bedroom condensation.
What Portuguese law says when windows are replaced
We read the text of Portaria n.º 138-I/2021 of 1 July in the Diário da República (the official gazette). In annex II, point 1.2.1, paragraph b), it says that in buildings undergoing renovation that involves installing or replacing the glazed envelope of main rooms, as long as it is not a major renovation, air-inlet solutions must be installed in the façade of those rooms.
Those solutions are one of two:
| Solution | Value asked in the ordinance |
|---|---|
| Opening in the façade | Minimum free area of 60 cm², per standard EN 16798-1 |
| Self-regulating vent | Nominal flow of at least 25 m³/h at 2 Pa or 75 m³/h at 20 Pa, per standard NP 13141-1 |
Paragraph c) exempts cases where the building already has an air-change rate of 0.50 per hour or more, calculated with the SCE Manual (the energy-certification method). Paragraph d) says that where there are technical or functional constraints, the project technician must adopt alternative solutions that do not put air quality and the healthiness of the rooms at risk.
We could not confirm whether this text, on its own, obliges someone who replaces two windows in a flat without a project. Treat it as the reference for good practice, and ask the installer. See also the guide on natural ventilation, vents and air bricks.
Air inlets: what they are and what not to do
The DHSC says that under UK building rules replacement windows must have trickle vents. That is the British regime, not the Portuguese one. The same document says vents should be controllable by the people living there, because uncontrollable ones tend to get blocked. It is worth checking they are not blocked. The FOPH advises against permanent ventilation through tilted windows while the heating is on.
How to tell whether the air is too humid
- Look at the glass. The FOPH says steamed-up windows are a sign of excess humidity in the air.
- Measure. A simple hygrometer gives relative humidity. The FOPH gives 30% to 50% in the heating season and no more than 40% on very cold days. See how to measure damp.
- Note where. Mould only on the window sealant joints is, according to the FOPH, a small area that does not cause problems, and replacing the sealant is simple. Mould in every room or on outside walls calls for a specialist.
- Wipe condensation early. The DHSC says most people can reasonably remove condensation and very small amounts of mould with a suitable cleaner, but that this is no substitute for fixing the cause. See mould on walls for method and safety.
A routine that reduces the problem
| Habit | Source |
|---|---|
| Air at least three times a day with a through draught, 5 to 10 minutes | FOPH |
| After a bath or cooking, ventilate or switch on the fan | FOPH |
| In old buildings with new windows and poor insulation, air more often | FOPH |
| Leave 10 cm between furniture and outside walls | FOPH |
| Keep air inlets unblocked and controllable | DHSC |
Bathing is the biggest daily culprit. See bathroom extractor fans. If the air stays humid anyway, a dehumidifier lowers humidity but does not bring fresh air.
What to ask the installer before you sign
- Does the window include an air inlet (a box or vent in the frame)? Of what kind, with what free area or flow, and is there a manufacturer data sheet?
- Is the inlet controllable by me?
- Does the frame carry CE marking, a declaration of conformity and a technical sheet? The ordinance asks for these for glazed-envelope elements in new or renovated buildings (point 2.1).
- Will the window be installed following the manufacturer’s instructions by someone qualified for the activity? That is what point 2.3 says.
- Is all of this written in the quote?
When it is not just ventilation
If mould returns in the same corner every winter, or the wall is damp from infiltration or ground damp, more ventilation will not fix it. See the types of damp in walls. If you plan to insulate walls from the inside, first read the risk described in internal insulation and condensation. The FOPH also recommends, before window repairs or insulation work, an assessment by a building-physics specialist or ventilation planner who looks at thermal bridges and ventilation equipment.
Paint only after the cause is dealt with. Painting over is not a repair: the FOPH gives the example of a damp wall merely painted with anti-mould paint, where mould and damp stayed.
What we could not confirm
- Whether Portaria n.º 138-I/2021 obliges an isolated window replacement in an existing home without a project.
- The text of standards EN 16798-1 and NP 13141-1, which the ordinance cites. We did not read them.
- The SCE Manual and the air-change calculation for a specific home.
- Portuguese legal limits for indoor humidity. The 30% to 50% figures come from the FOPH (Switzerland).
- The full WHO guidance on humidity and ventilation: we only read the press release, which says occupants should manage water use, heating and ventilation to avoid excess humidity.
- Prices of windows with air inlets.
Sources (read on 4 November 2026)
- Diário da República, series 1, no. 126, 1 July 2021, Portaria n.º 138-I/2021: points 2.1 and 2.3, annex II point 1.2.1 paragraphs a) to d), Table 10 (text extracted from the PDF and read).
- Swiss Federal Office of Public Health (FOPH), Bolor em espaços habitáveis, Portuguese version: causes, ventilation, relative humidity, furniture, the window-joint example, assessment before repairs.
- Department of Health and Social Care (UK), Understanding and addressing the health risks of damp and mould in the home: energy efficiency and ventilation, air inlets, condensation (first 100,000 characters read).
- World Health Organization (Europe), WHO publishes first indoor air quality guidelines on dampness and mould, 16 July 2009: press release. The full guidelines were not read.